(1) Field of the Invention
This invention relates generally to processing of polymer films, notably in the form of continuously moving webs, and specifically to cutting of such films or webs, generally in continuous operation and in longitudinal or machine direction.
(2) Description of the Prior Art
Various machines used for continuous production or processing of polymer films, e.g. winders of the type disclosed in U.S. Pat. Nos. 1,687,928, 2,915,255, 3,949,566 and in my U.S. Pat. No. 4,191,341, may require a continuous cutting operation to be performed at the moving polymer web, generally at a marginal area thereof and in longitudinal direction (parallel to machine direction), e.g. for continuously opening a blown polymer film hose at its sides so as to produce two separate polymer webs that can be wound up separately. Further, a wider polymer web may require division into a number of parallel strips, or a web may require longitudinal side portions to be cut away, e.g. after coating, etc.
Any such longitudinal continuous cutting operation requires prolonged cutting of polymer films, generally at relatively high speeds, and dulling of the cutting edge must be prevented or controlled if undesired tearing or rupturing of the polymer film is to be prevented.
Another type of cutting operation associated with production or processing of polymer films is transverse (to machine direction) cutting, e.g. when a length of web has been wound-up on a mandrel and the continuous web must be cut to end winding on a previous mandrel and to start winding on another mandrel. It will be understood that transverse cutting requires relatively less actual cutting time of a knife but dulling may still be a problem, notably when the knife edge is in contact with the surface of a roller.
Broadly, four types of mechanical cutting modes can be distinguished in polymer film cutting:
(a) press cutting, i.e. when the cutting edge is pressed onto the polymer film which in turn is supported by a surface or anvil;
(b) shear cutting, i.e. when two cutting edges interact upon the polymer film in the manner of shear blades;
(c) rotational cutting, i.e. when a circular knife is rotated at high rotating velocities in the general manner of a circular saw while simultaneously moving relative to the polymer film;
(d) slit cutting, i.e. when a sharp edge of a blade is contacted with an unsupported polymer film.
All of the above cutting modes require a relative linear movement between the polymer film and the edge at the cutting point; such relative linear movement is critical in the sense that no continuous cutting occurs in the absence of such movement, and will be termed "primary cutting motion" herein. Another or secondary motion may be superimposed upon the primary motion. For example, a common shear with its blades somewhat opened may be moved relative to a polymer film and cuts the latter without the usual secondary motion of opening and closing the shear.
Accordingly, it will be appreciated that press cutting and shear cutting may include a secondary cutting motion, e.g. rotation of a circular knife, in addition to the primary motion or linear movement; rotational cutting, by definition, includes both primary and secondary motion while slit cutting involves but primary motion.
Most devices used for continuous longitudinal cutting of polymer films are those developed in the paper industry, i.e. press cutting or shear cutting devices comprising rotatable circular knives which, in press cutting, are pressed onto a counter-roller having an extremely hard surface or, in shear cutting, cooperate with a second rotatable circular knife to form a shear edge; in either case, the circular knives used must be of a rugged construction, i.e. have a substantial thickness of several millimeters to support the stresses of coacting with the support roller or the second knife.
Circular knives for rotational cutting must be suitable for cutting at relatively high speeds of typically above 1000 RPM and require a rigidity that cannot be achieved with a blade thickness below the millimeter range.
In general, previous devices for longitudinal continuous cutting of polymer films have performed satisfactorily with many conventional polymer films; however, there is a growing tendency to include various additives in polymer films to improve or modify certain properties and some typical additives are very abrasive. As a consequence, rapid and, sometimes, uncontrolled dulling of the knives becomes a problem of increased importance.
The possibility of counteracting the abrasive action of polymer film additives by improving performance properties of conventional knives is limited, however, both for reasons of costs of material and maintenance. In this connection, the use of discardable knives has been considered and attempts have been made to use such easily replaceable blades, such as conventional razor blades of the type used in safety razors; the usable cutting life of such blades is limited, however; controlled placement of fresh edge portions into cutting position is difficult, if not impossible, in an automated arrangement and blade utilization is low.